1. Field of the Invention
The present invention relates to an optical transmitter-receiver and an optical fiber. More specifically, it relates an optical transmitter-receiver that is connected with single-core optical fiber, and the like.
2. Description of Related Art
FIG. 1 is an explanatory diagram of an outlined configuration of an optical communication system 100 for performing bi-directional communication using single-core optical fiber. This optical communication system 100 comprises two optical transmitter-receivers 103A, 103B each including a light-emitting element 101 and a light-receiving element 102, and a single-core optical fiber 104 for connecting the optical transmitter-receivers 103A, 103B. When the optical transmitter-receiver 103A transmits data to the optical transmitter-receiver 103B, the light-emitting element 101 in the optical transmitter-receiver 103A emits light. The light emitted from the light-emitting element 101 in the optical transmitter-receiver 103A is transferred through the optical fiber 104. The light-receiving element 102 in the optical transmitter-receiver 103B then receives it.
When the optical transmitter-receiver 103B transmits data to the optical transmitter-receiver 103A, the light-emitting element 101 in the optical transmitter-receiver 103B emits light. The light emitted from the light-emitting element 101 in the optical transmitter-receiver 103B is transferred through the optical fiber 104 used to transmit data from the optical transmitter-receiver 103A. The light-receiving element 102 in the optical transmitter-receiver 103A then receives it.
Such a technology for simultaneously performing transmission and reception between the optical transmitter-receiver 103A and the optical transmitter-receiver 103B using the single-core optical fiber 104 is referred to as “single-core bi-directional full-duplex optical fiber communication”, for example.
To perform the single-core bi-directional full-duplex optical fiber communication, it is required to provide an optical transmitter-receiver having a function for guiding the light emitted from the light-emitting element 101 to the optical fiber 104 and the light launched from this optical fiber 104 to the light-receiving element 102. As the optical transmitter-receiver having such the function, the optical transmitter-receiver having such a configuration as to use a beam splitter is known.
FIG. 2 is a plan view for showing an outlined configuration of a conventional optical transmitter-receiver 103 equipped with such the beam splitter. The optical transmitter-receiver 103 emits light from the light-emitting element 101 and splits it using a beam splitter 107 having a transmission coefficient of about 50% and a reflection coefficient of about 50%. The optical transmitter-receiver 103 then focuses the emitted light on an end face of the optical fiber 104 using a lens 108. The optical transmitter-receiver 103, on the other hand, focuses light reflected by the optical fiber 104 through the lens 108 used in the transmission and passes it through the beam splitter 107. The light-receiving element 102 then receives it. In FIG. 2, the light thus emitted is indicated by a solid line and the light thus received, by a broken line.
In the optical transmitter-receiver 103 using the beam splitter 107, the light thus emitted and received uses the same optical axis, so that the lens 108 can be arranged in the vicinity of the end face of the optical fiber 104 to focus the light emitted and received. This results in an increase in both of an efficiency of incident light from the light-emitting element 101 to the optical fiber 104 and an efficiency of receiving of it from the optical fiber 104 to the light-receiving element 102.
In the optical transmitter-receiver using the beam splitter 107, however, the light emitted from the light-emitting element 101 and reflected by the end face of the optical fiber 104 is focused through the lens 108 on the light-receiving element 102 and coupled to the light-receiving element 102. Thus, the optical transmitter-receiver has such a disadvantage that large crosstalk peculiar to single-core bi-directional optical fiber communication occurs.
FIG. 3 is an explanatory diagram of principles of crosstalk occurring. In FIG. 3, a letter, “S” indicates signal light from an optical transmitter-receiver, not shown in the figure. Alternatively, the beam splitter 107 reflects other signal light emitted from the light-emitting element 101. The lens 108 then focuses and enters it into the optical fiber 104 through its end face. The end face of the optical fiber 104, however, reflects a part of the light emitted from this light-emitting element 101. The part is then focused through the lens 108 on the light-receiving element 102 and coupled to the light-receiving element 102 because the signal light S and said other signal light use the same optical axis in propagation. This provides crosstalk N.
The following will describe an example for obtaining following Equation (1) for calculating an S/N (the signal light S and the crosstalk N) ratio of a conventional optical transmitter-receiver using a beam splitter.S=0.5aP2=0.5P2 andN=0.5×0.5bcP1=5.0×10−3P1S/N=100P2/P1  (1)wherein the calculation is performed on the basis of the following assumption:
P1: intensity of light emitted from light-emitting element;
P2: intensity of light emitted from fiber;
a: coupling efficiency of signal light with light-receiving element;
b: reflection coefficient of light being reflected by end face of fiber; and
c: coupling efficient of light returned from end face of fiber with light-receiving element.
It is to be noted that in the calculation, a transmission coefficient of the beam splitter is supposed to be 0.5 and a reflection coefficient of it, 0.5.
If a signal light is coupled to the light-receiving element totally, a=1 is given. If the optical fiber is made of fluorine-based plastic fiber having a refraction index of about 1.35, b=0.02 is given. If the light returned from the end face of the fiber is totally coupled to the light-receiving element, c=1 is given.
In single-core bi-directional communication in a giga-hertz frequency band, to achieve a bit error rate BER<10−12, generally S/N>10 is necessary, so that a tolerable loss is given by the following equation (2):P1/P2>0.1  (2)
According to Equation (2), only a loss of −10 dB is allowed a range from the light-emitting element to a light-launched end of the fiber. A loss of −3 dB is suffered through the beam splitter until light reaches an incident end of the fiber, so that the remaining loss of −7 dB is tolerable.
Assuming, for example, a case of laying down fluorine-based plastic fiber having a transmission loss of −4 dB/100 m, a flexure loss of 0.2 dB/90°, and a tolerable curvature radius R=20 mm, the fiber can be flexed only 15 times over a distance of 100 m. This inflicts a heavy restriction on laying down of the fiber, so that the requirement of S/N>10 cannot be met in such a laying-down environment that the fiber is always flexed 16 time or more, for example, thus proving a difficulty in single-core bi-directional communication in a giga-hertz frequency band.